It is known to use a retention device on rotating blades, for example, on a propeller of an aircraft engine, to help prevent radial separation of the blades from the hub on which they are mounted. It is also known to use a secondary retention device which acts to prevent the radial separation of the blade and hub in the event of a failure of the primary retention device.
One known propeller assembly which utilises such a secondary retention device is shown in FIG. 1. The propeller assembly 10 comprises a hub 12 and a blade 14 having an aerofoil 16 and root 18. The blade 14 is configured to rotate about axis 19 with hub 12 on bearings (not shown) so as to provide a thrust in the direction indicated by arrow 21. Bearings 24, 25 are provided such that the blade can be rotated about its longitudinal axis so as to provide a pitch control system. During manufacture of the propeller assembly 10, the root 18 is inserted into the hub 14 via an aperture 20 in the outer surface 22 of the hub 14 and the bearings 24 inserted between respective radially opposing faces 26, 28 of the hub 14 and root 18. The bearings 24 provide the rotational support required of the blade 14 and also act to prevent radial separation of the blade 14 and hub 12 during use. In this way, the bearings 24 act as a primary retention device.
The secondary retention device in the embodiment shown in FIG. 1 is a circular wire 32 which is fed into a channel formed from corresponding grooves in the facing surfaces of the hub 12 and the root 18. In the event of bearing failure, radial separation of the blade 14 and hub 12 is prevented by the wire.
Although the use of the wire 32 as a secondary retention device can be adequate, it relies on being correctly installed which requires a skilled operator and excessive amounts of time.
Further, because the wire 32 is neither an integral part of the hub 12 nor the blade 14, it is subjected to relative movement between these components which can lead to vibration and wear problems. A further difficulty with this system arises due to the concealed location of the wire which makes inspection more troublesome.
The present invention seeks to overcome some of the problems with known prior art.